1. Field of the Invention
This invention relates generally to a fuel cell system and, more particularly, to a fuel cell system that employs a battery/capacitor electrical energy storage system that eliminates the need for a DC/DC converter.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
Most fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental power source in addition to the fuel cell stack, such as a DC battery or a super capacitor (also referred to as an ultra-capacitor or double layer capacitor). The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery through the DC bus line.
In the hybrid vehicle discussed above, a bi-directional DC/DC converter is typically necessary to step up the DC voltage from the battery to match the battery voltage to the bus line voltage dictated by the stack voltage and step down the stack voltage during battery recharging. However, DC/DC converters are relatively large, costly and heavy, providing obvious disadvantages. It is therefore desirable to eliminate the DC/DC converter from a fuel cell vehicle including a supplemental power source.
There have been various attempts in the industry to eliminate the DC/DC converter in fuel cell powered hybrid vehicles by providing a power source that is able to handle a large fuel cell voltage swing over the operating conditions of the fuel cell stack. In one known system, an ultra-capacitor (also referred to as a super capacitor and a double-layer capacitor) is used as the supplemental power source. However, the ultra-capacitor is limited by how much it can be discharged because of its low energy content compared to the battery. Also, the ultra-capacitor requires a power device to ramp up the capacitor voltage at system start-up. Certain types of batteries have also been used to eliminate the DC/DC converter in vehicle fuel cell systems. However, these systems were limited by the ability to discharge the battery beyond a certain level. In other words, these types of batteries would be damaged as a result of large voltage swings on the DC bus line during the operation of the system.
U.S. patent application Ser. No. 11/112,103, titled DC/DC-Less Coupling of Matched Batteries to Fuel Cells, filed Apr. 22, 2005, and assigned to the Assignee of this application, discloses a proposed system that eliminates the DC/DC converter in a fuel cell hybrid vehicle. This system employs a matched battery whose voltage output is matched to the DC bus line over its entire voltage operating range. However, in this design, the battery state of charge (SOC) swing during vehicle operation may lead to a short battery life for current state of the art batteries, such as NiMH batteries. For example, the battery SOC swing may be between 20% capacity at its lowest discharge point and 80% capacity at its highest charge point, providing a 60% SOC swing. As the battery cycles over such a large SOC swing, the life of the battery may be significantly reduced.